[Asterisk-Users] T1-PRI deployment questions...

[Steven Critchfield]

On Thu, 2003-05-29 at 15:46, Steven Critchfield wrote:
> On Thu, 2003-05-29 at 15:06, Charles E. Youse wrote:
> > On 29 May 2003, Steven Critchfield wrote:
> > 
> > > On Thu, 2003-05-29 at 10:44, Charles E. Youse wrote:
> > > > B8ZS is required for PRI.  It's a digital service and can not handle the
> > > > loss of data required for AMI.
> > >
> > > I wasn't aware that AMI lost data. AMI just inverts polarity on the line
> > > for every other 1. B8ZS does the same thing but intentionally introduces
> > > errors on the line to maintain 1's density. Neither one is lossy.
> > >
> > 
> > AMI is lossy.  When the ones density in the signal is too low, AMI
> > will insert ones to ensure that the far end does not lose sync.
> 
> As I understand it, AMI is not lossy but will may cause problems due to
> not maintaining 1's density. 1's density is used to make sure both sides
> are synced up properly. B8ZS is AMI except that it introduces bipolar
> violoations to make sure the line doesn't stay in an off state for too
> long.
> 
> So B8ZS intentionally throws errors on the line in a known manner so as
> to make sure each side is in sync, but AMI does not care if the line
> goes all 0's for a while.
> 
> I still haven't been able to dig up any documentation to back up that
> AMI is lossy, just maybe prone to errors via slips.

I still can't find any reference to AMI being lossy, and can't find any
comments that show where a AMI circuit would introduce 1's to maintain
1's density. After reading a page describing test patterns and why they
use certain test patterns, it makes sense why AMI might not be usable
for a PRI though. 
http://www.electrodata.com/testpat.htm
In a PRI, since the signalling is in the D channel, and the consecutive
B channels could be completely clear, you could run into times with more
than 15 consecutive zeros. Although I need to do more looking at how D4
or ESF lays on top of a T1 signal. Anyways, with more than 15
consectuives zeros you no longer are within ANSI spec.

On a RBS circuit it would be less likely to fall too far out of spec
using AMI.
-- 
Steven Critchfield  <critch at basesys.com>